This application is based upon and claims priority of Japanese Patent Application No. 2001-040571, filed on Feb. 16, 2001, the contents being incorporated herein by reference.
1. Field of the Invention
The present invention relates to an optical waveguide device for trapping incident light in an area and transmitting the energy in a direction to output it.
2. Description of the Related Art
Splitting and combining of optical power (energy) are important basic functions in many optical waveguide devices including waveguides. In this case, reduction of energy loss between input and output and improvement of transmission loss uniformity among output ports are important objectives. Meanwhile, reduction of dimensions of optical waveguide devices is another requirement necessary to reduce manufacturing cost and facilitate use of the device in a broader range of applications.
Concrete examples of conventional optical waveguide splitting and combining devices are presented below.
First, a Multi-Mode Interference device (MMI) is cited as shown in FIG. 7.
The device is constituted by including an input waveguide 101 for converting incident light into a single optical mode, a plurality of output waveguides 102 for splitting power and to output it, and a waveguide 103 formed with a constant width along the propagation direction of light, connecting the input waveguide 101 and the output waveguides 102, and causing the propagation of a plurality of optical modes along the propagation direction of light.
Incident light, which is converted into a single optical mode by the input waveguide 101, is converted into a plurality of optical modes in the waveguide 103, and the power thereof is divided equally over the output waveguides 102 to be outputted. The field intensity in an end portion of the waveguide 103 on the side of the output waveguides 102 (in FIG. 7, marked by ellipse E) is distributed in a form having peaks at locations corresponding to the positions of the output waveguides 102. The distribution is shown in FIG. 7.
Next, a Star Coupler device is cited as shown in FIG. 8.
The device is constituted by including the input waveguide 101 as in the MMI device, a plurality of output waveguides 104 in a tapered form decreasing in width toward the output portions, radially provided for splitting power and to output it, and a passage 105 for connecting the input waveguide 101 and the output waveguides 104 and allowing light to freely propagate therethrough toward the output waveguides 104.
Incident light converted into a single optical mode by the input waveguide 101 freely propagates in the passage 105 and the power thereof is split in the output waveguides 104 to be outputted. The field intensity in an end portion of the passage 105 on the side of the output waveguides 104 (in FIG. 8, marked by ellipse E) is distributed smoothly in substantially a bell shape with a peak in the center region as shown in FIG. 8.
Next, a Y-branch device is cited as shown in FIG. 9.
The device is constituted by including the input waveguide 101 as in the MMI device, two output waveguides 106, each provided radially to split power and output it, and a tapered waveguide 107 for connecting the input waveguide 101 and the output waveguides 106 and having a very small taper angle to be adiabatic, that is, to make an optical mode invariable along the light propagation direction.
Incident light, which is converted into a single optical mode by the input waveguide 101, propagates along the waveguide 107 without changing optical mode, and the power thereof is split in the output waveguides 106 to be outputted. The field intensity at an end portion of the waveguide 107 on the side of the output waveguides 106 (in FIG. 9, marked by ellipse E) is distributed smoothly in substantially a bell shape with a peak in the center region as shown in FIG. 9.
However, the above-described conventional optical waveguide devices have the following disadvantages.
MMI devices are excellent in obtaining uniform optical coupling among the output waveguides 102 constituting the output ports, but the length of the waveguide 103 increases quadratically with the number of ports, which inevitably results in excessive device dimensions not practical for fabrication if a sufficient number of output ports is provided.
Star coupler devices can be reduced in size to be compact even with a number of output ports being provided, but require adjustment of the width of the passage 105 for obtaining uniformity in optical coupling among the output ports. In this case, extremely large width is required, which results in an increased length of the passage 105, and it is also necessary to provide tapers at each of the output waveguides 104, thus further increasing the size of the entire device.
The Y-branch devices realize an adiabatic state avoiding optical mode conversion, which requires reduction of the taper angle, and thus it requires extreme increases in length.
As described above, the conventional optical waveguide devices can satisfy the requirement of uniform optical coupling among the output ports, but it is extremely difficult for them to satisfy the requirement of reduction in size of the entire devices while satisfying the above requirement at present.
The present invention is made in view of the aforementioned problems, and its objective is to provide an optical waveguide device and an optical waveguide method with high reliability and high precision, which sufficiently meet both requirements of uniform optical coupling among the output ports and reduction in the entire device size, and being suitable for various kinds of useful applications.
The inventor reaches the modes of the invention presented below as a result of detailed study.
An optical waveguide device of the present invention comprises a single optical mode input waveguide, a plurality of output waveguides, and a tapered waveguide for connecting the aforementioned input waveguide and aforementioned output waveguides, gradually increasing in width from the aforementioned input waveguide toward the aforementioned output waveguides, and is characterized in that the aforementioned tapered waveguide has a sufficiently large taper angle so that optical power guided by lower order optical modes couple into higher order optical modes while the light propagates along the tapered waveguide.
Here, it is preferable that the width at a narrow end of the aforementioned tapered waveguide is larger than the width of the aforementioned input waveguide, but is set to a value small enough to support a discrete spectrum of a certain number of optical modes.
Further, it is preferable that the aforementioned tapered waveguide has a linear taper plane in section made by optimizing the taper angle by a numerical analysis method so that a field intensity profile at the wide end of the tapered waveguide is maximally flat.
Furthermore, it is preferable that the aforementioned output waveguides are provided in such a direction that the axes thereof point to the narrow end of the aforementioned tapered waveguide.
Further, it is preferable that the aforementioned output waveguides are placed on a wave front of an electromagnetic wave at the wide end of the aforementioned tapered waveguide.
Still further, it is preferable that each of the aforementioned output waveguides has a width optimized by a numerical analysis method to obtain substantially equal power coupling efficiency.
Furthermore, it is preferable that the aforementioned output waveguides are tapered, gradually increasing in width toward the wide end of the aforementioned tapered waveguide.
In this case, it is preferable that each of the aforementioned output waveguides has a sufficiently large taper angle so that higher order optical modes couple into lower order optical modes while the light propagates along each of the output waveguides.
Further, it is preferable that the aforementioned output waveguides are in a bent form.
Furthermore, the present invention also relates to an optical waveguide method, characterized by including the steps of converting incident light into a single optical mode, and after guiding the light into a waveguide having a sufficiently large taper angle such that lower order optical modes couples into higher order optical modes, splitting power thereof into a plurality of powers to output them.
In the optical waveguide device of the present invention, the waveguide has a non-adiabatic structure for causing mode conversion of transmitted light, thus making it possible to obtain uniform optical coupling among the output ports. Further, in addition to this, the waveguide has a sufficiently large taper angle such that lower order optical modes couple into higher order optical modes to achieve the aforementioned non-adiabatic structure, and therefore it is not necessary to increase the length of the waveguide as in the case of an adiabatic structure, thus making it possible to reduce the entire device in size even if further uniform optical coupling is intended with many output ports being provided. Specifically, it becomes possible to provide many output ports, to increase uniformity of optical coupling, minimize an increase in device size following this, and sufficiently meet the requirement for reduction in size of the device.
More specifically, when the tapered waveguide is constructed to have the linear taper plane in section, the length of the waveguide only increases proportionally to the number of output ports. Accordingly, as the number of output ports is increased, relative reduction in the device size becomes more remarkable, and thus the present invention is useful in realizing a compact optical waveguide device with more uniform optical coupling.